In late October the Italian Space Agency's BeppoSAX (Satellite for X-ray Astronomy) satellite had new attitude control software - Extended Science Modes 2 (ESM2) - uploaded to it that allowed scientific observation to recommence.

The initial design of the attitude control subsystem (AOCS) of the BeppoSAX astronomical satellite, built by Alenia Spazio for the Italian Space Agency (ASI) and orbited on April 30, 1996, foresaw the use of three permanently active gyroscopes and three reserve ones.

However, the limited life of these units, already demonstrated on other satellites, led Alenia Spazio to design and develop software that guaranteed the scientific pointing, using only one gyroscope (ESM1). This software was used up to the degrading of the last gyroscope, in September of this year, necessitating a gyroless reconfiguration of the attitude control software.

Gyroless pointing techniques have already been adopted on other satellites (Rosat, Soho and, most recently, ERS2), but in missions with different characteristics to BeppoSAX that requires greater complexity and autonomy to manoeuvre.

The satellite, managed from Earth and visible to the Malindi station in Kenya for only 10 minutes per orbit, is able to carry out a variety of programmed pointing to sources indicated by the scientists. Furthermore, it can detect Gamma Ray Bursts in the Universe, rapidly pointing its instruments in the direction of the burst to study its passage through the X-band.

Alenia Spazio's new software, ESM2, principally uses the three on-board Star Tracker sensors and orientates the satellite through the reaction wheels. It can maintain the scientific pointing using only one star sensor when the other two are blocked by the Earth.

In the case of a contemporary blocking of all three star sensors, ESM2 uses the Sun sensor and the magnetometer (much less accurate), automatically reacquiring an inertial attitude when one star tracker becomes available again, calculating and correcting the error through small redirections guided by a mini-star catalogue loaded on board.

The manoeuvres are made around the satellite's axis using one star sensor tracking two stars, or around any axis using the Sun sensor and the magnetometer.

The new software works using algorithms based on robust logic to manage the sensors and auxiliary functions such as an orbital propagator and a model of the Earth's magnetic field.

The flight test of ESM2 has proved that it can keep the BeppoSAX mission profile unchanged, meeting all the pointing requirements of the scientific instruments and carrying out a wide range of manoeuvres to reach the sources to be observed: another success for this Alenia Spazio-developed satellite whose use continues to be sought by scientists around the world.

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